The nonapeptide of the formula EQU Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu I
is a compound known under the abbreviation DSIP (delta sleep inducing peptide) (see, for example, Monnier et al., Experimentia 33, 548-552 [1977] or Schoenenberger et al., Proc. Natl. Acad. Sci. USA 74, 1282-1286 [1977]), having sleep-inducing properties.